1. Field of the Invention
The instant invention relates to a full mold casting process for a differential case member with cast-in bolt holes in which a complete positive model, made of a thermally decomposable material, is placed into a molding box filled with sand. The sand may be compacted by any suitable method such as shaking, and the casting metal is poured on the thermally decomposable model. The model is decomposed by the casting heat.
2. Description of Related Art
To produce castings, molds consisting of two halves are generally used.
However, this division of the molds is only possible within certain dimensional tolerances, and as a consequence, defects may occur within the mold joint due to offset burrs, sand washouts, metal penetrating into the sand mold, charred portions of sand, and the like. All these defects appear on the casting and must be eliminated by grinding, stripping, knocking, sawing, or the like.
Moreover, the prior art technique related to differential case members requires a machining process for the bolt holes where the bolt holes are machined into the casting.
To avoid extensive manual tasks, a so-called evaporative or full mold casting process has been proposed whereby a pattern is formed of an evaporable foam material, such as polystyrene, and is identical to the configuration of the metal part to be cast. The pattern is placed in a mold and a flowable material such as sand is introduced into the mold and surrounds the pattern as well as filling the cavities in the pattern. In the casting process, molten metal is introduced into the mold and the heat of the molten metal will vaporize the foam material with the vapor being trapped within the interstices of the sand, while the molten metal will fill the voids created by vaporization of the pattern to provide a cast metal part which is identical in configuration to the evaporable foam pattern. When such an undivided model is used, a casting without burrs is produced.
It has been found, however, that not all types of models can be molded in this manner and it is especially difficult to satisfactorily embed moldings with dome-shaped cavities, such as one-piece differential case members and similar shapes, in the sand because the sand does not rise to fill these cavities. It is, therefore, a disadvantage of this known lost foam casting process that such moldings cannot be molded and cast in this process in an efficient and effective manner but must be produced with batch cores or in the conventional molding process, in several parts. As an alternative, attempts have been made to distribute sand by an extreme amount of shaking of the molding flask. This shaking can cause damage to the foam pattern causing dimensional inaccuracies. In addition, the excess shaking will also increase the cost of the part due to the longer cycle times for filling the flask and excessive wear on the molding machine. All molds in which the sand would have to rise into such cavities as well as into communicating pipes are, therefore, inefficient and ineffective with the known full mold casting process for one-piece differential case member and the like.
The need therefore exists for a method of making a differential case or other similar article using lost foam casting whereby sand is allowed to flow into the interior of the part where the sand would otherwise not have access.
Moreover, the need exists for a process that eliminates the need to separately machine the bolt holes into the casting of a differential case.